When ready to molt, the crab "cracks" its shell open from the back and then backs out.

Crabs molt about 27 times throughout their lives. Young crabs molt very frequently--a zoea molts seven times with only a few days between each molt, but as the crab grows older the time between molts lengthens. After a crab reaches maturity the next molt can even be a couple months away. Food availability and environmental conditions can slow the molting process.

Without its old hard shell (which, once discarded is called a "slough"), the crab is now temporarily a soft-shelled crab. The new exoskeleton is there, but it is still very soft.

After shedding its old shell, the crab first expands its new shell by pumping water into its body. After that, it takes about 72 hours (three days) for the soft shell to harden. Until the shell is hard again, the crab is very vulnerable and has to hide from predators.

Molting stages

The molting cycle has different stages.

In general, there is the pre-molt stage when the crab is getting ready to molt within a few days.

Then there is the actual molting activity(ecdysis), which can last up to a few hours.

Next is the post-molt stage, during which the crab is expanding it's shell and avoiding predators.

The last stage is the intermolt stage, during which a crab is feeding and continues to increase in tissue mass underneath the exoskeleton.

The various stages of the molting cycle are indicated by external and internal signs. The picture below show the gill raker (or scaphognathite, a structure on the crab's inside) and the swim paddle (visible from the outside), two structures that change in appearance during the molting cycle. The gill raker is a feather-like structure that brushes over the gills and distributes any moisture inside the crab over the gills to keep them moist so they can continue to receive oxygen.

The picture below shows how the gill raker (top series) and swim paddle change in appearance during the molting cycle. On the left, during the intermolt, the featherlike structures in both series contain tissue. Going towards the right, and moving forward in the molting cycle, the crab pulls this tissue into its body, out of the exoskeleton, which is visible as a dark (top series) or colored line (bottom series) inside.